In common rail fuel injection systems, a plurality of injectors are typically provided to inject fuel at high pressure into the cylinders of an internal combustion engine. Each injector includes an injection nozzle having a valve needle that is operated by an actuator so as to move towards and away from a valve seating and to control fuel delivery by the injector. It is known that an optimum exhaust emission condition may be achieved if the rise and fall of the injection rate, at the beginning and end of injection, respectively, is as fast as possible, necessitating fast movement of the injection nozzle valve needle. Indirect acting injectors typically do not provide a fast needle response as they rely on a servo valve to control operation of the valve needle. Direct-acting piezoelectric injectors, however, are known to provide a fast needle response. In a direct-acting piezoelectric injector the actuator acts directly on the valve needle through a hydraulic and/or mechanical motion amplifier. Our European patent EP 0995901 describes a direct-acting piezoelectric injector of the aforementioned type.
One disadvantage of direct-acting injectors is that they are relatively inefficient, electrically, due to the large amount of electrical energy that is required to produce sufficiently high needle lift. In addition to direct loss of energy, the life of the piezoelectric actuator is also compromised due to the large amounts of energy required to drive the actuator.
It is an object of the present invention to provide an injection nozzle that addresses the aforementioned problem so as to enable energy efficiency to be improved when implemented, for example, in a direct-acting piezoelectric injector.